A pneumatic tire typically includes a pair of axially separated inextensible beads having at least one carcass ply extending between the two beads. The carcass ply includes axially opposite end portions each of which is turned up around a respective bead and secured thereto. Tread rubber and sidewall rubber are located axially from and radially outward of, respectively, the carcass ply.
The bead area is one portion of the tire that receives considerable attention during the tire design process. Under conditions of severe operation, the stresses in the bead area can be especially problematic, leading to separation of adjacent components. In particular, the ply turnup ends are prone to separation from adjacent structural elements of the tire. For this reason, a variety of structural design approaches have been used to manage separation of the tire elements in the bead area. These design approaches typically include adding various polymeric materials within the bead area to increase strength and rigidity. In many tire bead designs, one or more fillers or apexes will be disposed between adjacent components. For instance, an apex might be positioned immediately radially outward of the bead and between the carcass main portion and the turnup portion. A second apex might be positioned between the turnup portion and the tire side wall. Many bead designs include additional elements, such as chafers, chippers, toe guards, and clamping members, all designed to improve the durability of the bead area.
Bead area designs including the above-mentioned strengthening elements have some drawbacks, such as increased material costs as well as increased tire weight. To address these drawbacks, tire manufacturers continue to strive for bead area designs that have reduced weight and volume characteristics, yet provide high bead area durability. For certain tire markets, such as the radial light truck (RLT) tire market, manufacturers have made some progress in providing bead area designs with reduced weight and high durability. By way of example, in tires according to U.S. Pat. No. 5,524,688, which is assigned to the assignee of the present invention, a bead area having a locked bead construction includes a turnup portion that wraps around a substantially pentagonal bead without an apex between the main portion and the turnup portion and without a chipper or clamping member. This construction therefore eliminates many of the structural elements in many prior bead area designs, thereby reducing the weight but still providing high bead area durability.
For pneumatic tires used under relatively heavier loads, such as those for the radial medium truck (RMT) tire market, the volume of the various strengthening elements are typically increased to support the increased loads, thereby further increasing the material costs and weight of the bead area. Consequently, as with the RLT tire market, manufacturers desire bead area designs having reduced volume and weight characteristics that further provide high bead area durability. However, applying the teachings and techniques of the RLT tire market, such as that found in U.S. Pat. No. 5,524,688, to tires designed for heavier loads, such as RMT tires, often results in unacceptable stress concentrations in the bead area which may lead to ply separation and tire failure.
Thus, there continues to be a need for a simplified bead area design for heavier load radial tires that reduces the weight of the bead area yet provides high bead area durability.